valence
Practice
Oxidation and reductionNational 2024

What is ΔG∘\Delta G^\circ at 298 K for the disproportionation of PbO(s)\ce{PbO(s)}?

2 PbO(s)→PbOX2(s)+Pb(s)\ce{2 PbO(s) -> PbO2(s) + Pb(s)} ΔG∘\Delta G^\circ = ???

Half-reaction E∘E^\circ, V

PbOX2(s)+4 HX+(aq)+4 eX−→Pb(s)+2 HX2O(l)\ce{PbO2(s) + 4 H+(aq) + 4e- -> Pb(s) + 2 H2O(l)} 0.671

PbO(s)+HX2O(l)+2 eX−→Pb(s)+2 OHX−(aq)\ce{PbO(s) + H2O(l) + 2e- -> Pb(s) + 2 OH-(aq)} –0.580

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Explanation

draft

Step 1: Set up the two half-reactions with electrons balanced

PbOX2(s)+4 HX+(aq)+4 eX−→Pb(s)+2 HX2O(l)\ce{PbO2(s) + 4H+(aq) + 4e- -> Pb(s) + 2H2O(l)}, E1∘=0.671 VE^{\circ}_{1}=0.671\text{ V}

PbO(s)+HX2O(l)+2 eX−→Pb(s)+2 OHX−(aq)\ce{PbO(s) + H2O(l) + 2e- -> Pb(s) + 2OH-(aq)}, E2∘=−0.580 VE^{\circ}_{2}=-0.580\text{ V}

We need 2 PbO(s)→PbOX2(s)+Pb(s)\ce{2PbO(s) -> PbO2(s) + Pb(s)}, i.e. one PbO is oxidized to PbO2 and one PbO is reduced to Pb. Reverse half-reaction 1 (oxidation) and double half-reaction 2 (reduction) so electrons cancel (4 each):

  • Oxidation: Pb(s)+2 HX2O(l)→PbOX2(s)+4 HX+(aq)+4 eX−\ce{Pb(s) + 2H2O(l) -> PbO2(s) + 4H+(aq) + 4e-}, ΔGA=+4F(0.671)=+258.98 kJ\Delta G_{A} = +4F(0.671) = +258.98\text{ kJ}
  • Reduction (×2): 2 PbO(s)+2 HX2O(l)+4 eX−→2 Pb(s)+4 OHX−(aq)\ce{2PbO(s) + 2H2O(l) + 4e- -> 2Pb(s) + 4OH-(aq)}, ΔGB=2×[−2F(−0.580)]=+223.84 kJ\Delta G_{B} = 2\times[-2F(-0.580)] = +223.84\text{ kJ}

Step 2: Add and simplify

Adding gives:

2 PbO(s)+Pb(s)+4 HX2O(l)→PbOX2(s)+2 Pb(s)+4 HX+(aq)+4 OHX−(aq)\ce{2PbO(s) + Pb(s) + 4H2O(l) -> PbO2(s) + 2Pb(s) + 4H+(aq) + 4OH-(aq)}

Cancel one Pb and note the 4 HX+\ce{4H+} and 4 OHX−\ce{4OH-} must combine back to 4 HX2O\ce{4H2O} to give the target reaction (2 PbO(s)→PbOX2(s)+Pb(s)\ce{2PbO(s) -> PbO2(s) + Pb(s)}). This neutralization step is not free — it has ΔG∘=RTln⁡Kw\Delta G^{\circ} = RT\ln K_w per mole of HX2O\ce{H2O} formed.

ΔGneutralization, 1 mol∘=RTln⁡(10−14)=(8.314)(298)(−32.24)=−79.9 kJ\Delta G^{\circ}_{\text{neutralization, 1 mol}} = RT\ln(10^{-14}) = (8.314)(298)(-32.24) = -79.9\text{ kJ}

Since 4 mol of water are formed: ΔGneut=4(−79.9)=−319.6 kJ\Delta G_{\text{neut}} = 4(-79.9) = -319.6\text{ kJ}

Step 3: Total

ΔG∘=ΔGA+ΔGB+ΔGneut=258.98+223.84−319.6≈163 kJ mol−1\Delta G^{\circ} = \Delta G_{A} + \Delta G_{B} + \Delta G_{\text{neut}} = 258.98 + 223.84 - 319.6 \approx 163\text{ kJ mol}^{-1}

This matches choice (A).

A

Correct: properly reverses/doubles the half-reactions to balance electrons, then adds the 4H++4OH−→4H2O4H^{+}+4OH^{-}\rightarrow 4H_2O neutralization free energy (4RTln⁡Kw≈−319.64RT\ln K_w \approx -319.6 kJ), giving ΔG∘≈163\Delta G^{\circ}\approx163 kJ.

B

Skips the proper half-reaction balancing entirely: treats the two given standard potentials as a simple cell (Ecell∘=E2∘−E1∘=−1.251E^{\circ}_{cell}=E^{\circ}_{2}-E^{\circ}_{1}=-1.251 V) and uses n=2n=2 (from the PbO/Pb half-reaction) in ΔG∘=−nFE∘\Delta G^{\circ}=-nFE^{\circ}, giving −2F(−1.251)=241-2F(-1.251)=241 kJ — ignoring that the two half-reactions are written in different media (acid vs. base) and are not directly subtractable.

C

Correctly reverses/doubles the half-reactions and sums ΔGA+ΔGB=482.8\Delta G_A+\Delta G_B=482.8 kJ, but forgets to scale the water-formation correction by the stoichiometric factor of 4, subtracting only one RTln⁡KwRT\ln K_w term (−79.9-79.9 kJ) instead of four, giving 482.8−79.9≈403482.8-79.9\approx403 kJ.

D

Uses the same naive cell-potential subtraction as choice B (Ecell∘=−1.251E^{\circ}_{cell}=-1.251 V) but applies n=4n=4 (from the PbO2/Pb half-reaction) instead of n=2n=2, giving −4F(−1.251)=483-4F(-1.251)=483 kJ; this is simply ΔGA+ΔGB\Delta G_A+\Delta G_B with no correction at all for the acid/base mismatch (missing the water autoionization term).

Concept · Combining half-reactions across different media using Hess's law and the water autoionization constant (Kw) correction

USNCO 2024 National Part I exam, question 42. © American Chemical Society; reproduced for non-commercial practice.